Simply put, it's a limitation of the current workflow capability.
You can do the opposite, which is update a parent from a child, but only if it's a Master Detail relationship.
Have you looked at your model and requirement(s) closely, though? It's likely you could bring the values from your parent object down to the child object through one ore more formula fields.
Also, keep an eye out for the eventual release of "headless flow" or "flow triggers", which aims to drastically improve the workflow capability.
You can do the opposite, which is update a parent from a child, but only if it's a Master Detail relationship.
Have you looked at your model and requirement(s) closely, though? It's likely you could bring the values from your parent object down to the child object through one ore more formula fields.
Also, keep an eye out for the eventual release of "headless flow" or "flow triggers", which aims to drastically improve the workflow capability.